in BlueSpice 5 the User Manager is merged into the Permission Manager’s custom mode. According to its reference here, you can add the following line to your post-init-settings.php:
This may be enabled in paid versions of BlueSpice but it is not in BlueSpice free
Also, if the UI has a button “Edit Groups” then that button should either work as expected or be removed
In the context of the “User manager” page, I believe that this is a sense making name for such a button.
For the Permission Manager config I mentioned, I double checked with a fresh installation of the free edition of BlueSpice 5.1.4 and 5.1.3 (from GitHub - BlueSpice-Wiki/bluespice-free-release ), and confirmed that it does bring the “custom” preset (together with the group managing functions) to me. Please leave a message if the mentioned config does not work for you.